I can't tell you how often I see queries in my inbox that start "I self-published my book ___, but I am frustrated/underwhelmed with the sales and now I need an agent to take it to the next level."
They need someone to boost sales for them and teach them how to market. A publicist. Not an agent.
Agents do more marketing all the time as the landscape shifts (me more than many--I like it). But this is done for clients--people for whom we have sold a book. Any marketing assistance would be supplemental to the function of selling the book (the only thing, incidentally, for which an agent is traditionally paid--on commission). If you've already gotten the book published, you've essentially done the agent's primary job.
As I said in the previous posts in our self-publishing series, legacy/traditional publishing and self-publishing are different paths. If you're looking to move between these paths make sure you're realistic about how much you expect the other system to bend over backwards to accommodate you.
Agents functioning primarily as a publicist is just not going to happen. We sell: foreign, print, audio, etc. etc., and we want to do so for self-published authors too. But you've got to sell those copies yourself first.